Samsung tablets are nutorious for needing the correct power adapter (block) in order to charge. You will find that in some cases the tablet is charging even if the red x is present. Plug your device into the desired power adapter and wait an hour. Next look at your battery percentage. If the battery has charged fast enough for your standards than keep it. But if its charging really slow or not at all then I'd suggest getting a new power adapter. I would suggest getting one from amazon.com. in order to ensure that you buy the correct power adapter, look on your old power adapter to see what the model number is and match it to the one you want to purchase. If you dont have the old power adapter then search for "(device model #) power adapter"
To find your devices model number go to:
Settings/about device/model number
When looking on amazon make sure that the power adapter you are going to purchase says that it is for your device. Sometimes there are products for different devices in the search results.